e53 4.4i gear problem, please help

Hello everyone, the thing is that car goes to sport regime by itself after couple of hours ride, and gas pedal becomes very sensitive, no matter how slowly I push, it reacts like a beast, and I can't help it unless stopped and cooled down. I showed my car to local mechanics and was told that problem is gear oil overheating because of gear oil radiator, which I already replaced but the problem remain the same. When I bought my X5 i changed the gear oil btw, I don't know if it matters or not. if u got any clues about this problem please help, thanks beforehand...

(sorry if I use improper names and words, it's because of I'm not a native speaker and moreover not familiar with autophrases)

4.4 - I am new to my 4.4i X5 as well, you sparked my curiosity on "sports regime" - mind elaborating on this feature....

saying "sports regime" I mean sport mode - the feature of semi-automatic and automatic transmission to change gears on high RPM-s, hope I satisfied ur curiosity )) any help in solvation of this problem will be appreciated ...